The invention generally relates to the field of electrooptic devices, and more particularly, to the biasing system of integrated electrooptic devices, for example, waveguide modulators made of electrooptic material, such as lithium niobate.
Transmission of data using optical carriers enables very high bandwidths and numbers of multiplexed channels with low signal loss and distortion. A coherent laser light beam is amplitude modulated with a data signal, and propagates to a remote receiver either directly through the atmosphere, or via a system of optical fibers and repeaters. The light beam may advantageously be modulated with electrical signals in the microwave frequency range using an electrooptic modulator such as a Mach-Zehnder modulator or an optical directional coupler. These modulators may be optical intensity modulators, switches, phase or frequency shifters, polarization transformers, wavelength filters, and the like. A class of these modulators are made of ferroelectric materials, such as z-cut lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3).
An electrooptic modulator based on a Mach-Zehnder interferometer generally includes a substrate having an optical waveguide formed therein having two arms or branches that extend generally in parallel with each other and have approximately equal lengths. The index of refraction of the material in the waveguide is higher than the index of refraction of the material of the substrate. In the absence of an applied electrical bias voltage, an input optical beam produced by a laser or the like applied to the waveguide divides equally between the branches into two beams, which propagate through the separate branches and recombine at the optical output of the waveguide in phase with each other to produce an optical output signal that is essentially similar to the optical input signal.
Optical modulators use the electrooptic effect to modulate an input light wave in amplitude according to an input signal. These optical modulators are designed to have an operating point that is optimally set by the application of an appropriate predetermined bias voltage. When a bias voltage is applied to one branch of the waveguide, it causes the indice of refraction of that branch's material to vary due to the electrooptic effect, such that the effective optical length of that branch varies as compared to the branch that is not subjected to the bias voltage.
For example, at a bias voltage known in the art as V.sub..pi., the effective optical lengths of the branches vary to such an extent that the optical signals emerging from the branches are 180.degree. out of phase with each other. Therefore, amplitudes of the signals combine to cancel each other out and produce a zero output at the optical output. For most optical communication applications, the modulator is biased at a voltage V.sub..pi. /2. Device instabilities and environmental effects, especially temperature variations, however, cause the operating point to drift, and require constant readjustment to maintain the proper operating point.
For this reason, the temperature dependence of modulators has been viewed as an operational shortcoming to be minimized. For example, to compensate for variations due to temperature, prior art devices use a feedback control circuit to provide more reliable control of the bias voltage applied to the optical modulator. See, e.g., U.S. Pat. No. 5,742,268 (Noda) and U.S. Pat. No. 5,003,624 (Terbrack et al.), the specifications of which are incorporated herein by reference. Further, in the case of z-cut LiNbO.sub.3 crystals, which are particularly sensitive to temperature variations, prior art constructions reduced temperature sensitivity by coating the modulator with an insulating film. See, e.g., U.S. Pat. No. 5,388,170 (Heismann et al.), the specification of which is incorporated herein by reference.
The stability of lithium niobate modulators, in particular, has been observed based on the application of a voltage, which is monitored throughout the lifetime of the device under test. It has been observed that over long periods of time the absolute magnitude of the bias voltage increases in an approximately linear fashion. This increase in starting bias voltage is attributed to a screening effect caused by the creation of an electric field in the substrate material when voltage is applied. In practice, as the bias voltage increases, it becomes increasingly difficult to control the modulator. As a result, the generated bias voltage becomes fixed at the upper or lower limit, which causes the modulated light output from the optical modulator to be distorted. To prevent this distortion, the end of the modulator life is typically set at the point at which the starting bias voltage reaches the maximum value of the power supply or simply the point at which the starting bias voltage reaches a multiplicative factor of the initial starting bias voltage. For example, the end of life can be defined when the starting bias voltage of a modulator having an initial starting bias of four volts reaches eight volts.
Further background material concerning the physics of ferroelectric crystals may be found in a number of references including books by Charles Kittel, Introduction to Solid State Physics,John Wiley and Sons, Inc., New York (1971) and Ivan P. Kaminov, An Introduction to Electrooptic Devices, Academy Press, Inc., Orlando (1974), both of which are incorporated herein by reference. Additional information on the processing of LiNbO.sub.3 may be found in the book chapter by S. K. Korotky and R. C. Alfeness, titled: "Ti:LiNbO.sub.3 Integrated Optic Technology" in Integrated Optical Circuits and Components, 169-227, Marcel Dekker, Inc., New York (L. D. Hutcheson ed. 1987), which is also incorporated herein by reference.